Selective transplantation of purified erythrocyte precursors will be evaluated as a method to repopulate erythrocytopoiesis, when either precursor cells are depleted or a genetically inherited abnormal erythrocyte population exists. Current attempts to correct hemopoletic defects by transplantation utilize complete bone marrow and are complicated by rejection, life-threatening immunosuppression, and a graft-vs.-host syndrome. The current program proposes to answer these problems by supplying a pure population of erythrocyte precursors incapable of initiating graft-vs.-host reactions or stimulating histocompatibility mediated rejection. Preliminary studies performed in our laboratory strongly suggest that human and canine fetal livers can supply a pure population of erythrocyte precursors. In addition, these early studies indicate that such precursor cells are incapable of either initiating or stimulating blast transformation in one-way mixed lymphocyte clultures. The proposed study is designed to accomplish the following specific objectives: 1) Determine the number of purified erythrocyte precursor elements that may be obtained from fetal dog and human livers at varying periods of gestation. 2) Evaluate both canine and human fetal liver erythrocyte precursors for the emergence and maturation of histocompatibility antigejs and the presence of potentially immunocompetent cells. 3) Establish a canine model test system to evaluate the feasibility of employing fetal liver erythrocyte precursors for selective transplantation repopulation of erythrocytes. 4) Investigate the relative roles of erythrocyte blood group isoantigens and histocompatibility antigens in the rejection of erythrocyte precursors and mature cells. 5) Determine if a purified suspension of transplanted erythrocyte precursor can initiate graft-vs.-host syndrome in recipient dogs. 6) Evaluate the capability of antithymocyte antisera to reduce both immune rejection and a graft-vs.-host syndrome in dogs undergoing selective erythrocyte transplantation.